A "highly suspicious" new case of foot and mouth disease was found last night in pigs at a farm in Northumberland. It could be the source of the original outbreak of the virus, which has thrown the British farming industry into fresh crisis.

The development came as the countryside began to close down, with appeals for ramblers and horse riders to stay away, markets shutting up shop and the suspension of all hunting for a week.

The suspected outbreak in Northumberland, at Heddon-on-the-Wall, was traced by the Ministry of Agriculture because the farm had delivered pigs for slaughter to Cheale Meats at Little Warley in Essex, where the virus was first identified on Monday night. When vets arrived at the farm some of the remaining pigs were sick with symptoms similar to foot and mouth but no confirmation has yet been made.

Another case of the disease was confirmed in cattle at Great Warley, near Brentwood, inside the 10-mile exclusion zone around the abattoir. The zone had been extended from five miles early yesterday as a precaution. The new case is likely to have been spread by the wind.

All movements of animals, lorries and visitors to the Essex slaughterhouse and its neighbouring farm over the past three weeks were being traced yesterday. The investigation involves hundreds of inquiries as far away as Northern Ireland and Scotland.

The president of the National Farmers Union, Ben Gill, warned: "The outbreak of foot and mouth could become a disaster if people travel into rural areas. Everyone must be vigilant and journeys into the countryside where there is livestock should not take place."

Postmen were asked to leave mail at farm gates and farmers were asked to place disinfectant-soaked straw at access points to their land. At agricultural merchant CWG in Bury St Edmunds, Michael Walpole had run out of disinfectant by lunchtime yesterday: "We have had no notification from Maff [the Ministry of Agriculture, Fisheries and Food] about what to do but we have sold out. I have also pulled all reps from farm calls."

All hunting by foxhounds, harriers, beagles, and staghounds was suspended for a week after meetings between masters of hunts. The British Horse Association advised riders everywhere to keep to the roads and off farmland.

In the Peak District, Bakewell livestock market cancelled a planned sale on Monday. A partner in the market, Alistair Snedden, said the outbreak could have far-reaching consequences for farmers: "I think the financial and emotional strain could quite simply be too much for some of them."

The ban on all animal exports, which was imposed on Wednesday night and is costing more than £1m a day, will be in force until Britain controls the spread of the disease, the European commission said. If the source of the contamination were pinpointed the ban could be limited to that region, a spokeswoman said.

Tony Blair, on a visit to Canada and the US, said the outbreak was "the very last thing that farmers need" in the aftermath of BSE. "This is a very, very tough time for them," Mr Blair said in Ottawa. "We have to contain the outbreak. We've taken the action necessary to do so, and we will look into the consequences for individual farmers, of how we help them, at a later time."

As fears spread to the continent, the French government ordered checks on the wind direction across the Channel.

The British Veterinary Association said it and Maff had put the country's 14,000 vets on "red alert". "Extreme surveillance is needed," said Eifion Evans, past president of the BVA.

Main points

 Ramblers advised to cancel all walks in affected counties and stay away from farmland throughout Britain.

 Riders told to keep to roads off bridleways and farmland.

 All hunting voluntarily suspended for a week.

 Farmers asked to place disinfectant-soaked straw at entrances to their farms.

 Postmen delivering to infected farms told to hand mail over at gates.